The use of high-brightness LEDs (light emitting diodes) in lighting applications is growing rapidly as a result of inherent benefits to LED technology such as long lifetimes, good efficiency, and the ability to use non-toxic materials. However, retrofitting existing applications with LED fixtures often requires compatibility with the large installed base of dimmers, particularly leading-edge triac-based dimmers. Because these dimmers were commonly designed for current levels much higher than those consumed by LED applications, many problems occur with existing LED driver solutions.
Triac-based dimmers function by allowing current to pass during a fraction of the half-cycle of the input AC mains voltage. One of the most common types of triac dimmers is the leading-edge type, which initially turns on at some point past the zero-crossing of the AC waveform (in both the upward direction and the downward direction), and then turns off at the next zero-crossing.
Most leading-edge triac-based dimmers were designed for use with incandescent light bulbs. In order to turn on and power the bulb, the triac requires a latching current to flow through the load. Subsequently, to maintain the triac's on state until the next AC zero-crossing, a lesser holding current must be present. This triac behavior matches well with the strongly positive temperature coefficient of incandescent bulbs. When cold and unpowered, an incandescent bulb presents a filament resistance which is a fraction of its value when powered. As current and power dissipation increase, temperature and hence resistance increase greatly. By its nature, the incandescent bulb provides a large latching current at the time of turn on, and maintains a lesser holding current while lit. Since one of the advantages of LED-based incandescent bulb replacements is power efficiency, it naturally draws less current than the hot incandescent bulb, and much less than the cold incandescent bulb.
When powered with triac-based dimmers, the performance of traditional LED driver ICs suffers in several ways. First, the driver efficiency generally falls well short of the desired targets. Even with the degraded efficiency due to a bleed of either constant current or constant resistance, many driver solutions fail in terms of gross functionality with digitally-controlled triac-based dimmers, which require low load impedance even in the standby state, when the dimmer is not explicitly powering the driver yet needs to keep standby circuits alive.
When trying to address these concerns, existing solutions can grow substantially in size, complexity, and power consumption. These concerns are addressed by the present invention.